[Coco] New CoCo 3 - RF Out problem

[Rogelio Perea]

On Sun Oct 25 2015, Chad H wrote:

Anyone have idea what could be going on here?  Simple fix I hope.   I could
> still use it in Line IN mode but I would rather have the CoCo FULLY
> functional if possible...
>

Recently I fixed a CoCo 3 that had good RGB and composite video out but no
RF. Sound was also garbled.

The problem was with the 8vdc regulating circuit (IC36 on the Service
Manual schematic), two electrolytic caps (C63 was one of them) were bad
preventing proper voltage to the RF modulator *and* audio amplifier (Q4).
Check those voltages out, if not 8vdc then troubleshoot the power supply
section around the 78L08 regulator IC (IC36).

I sort of figured out the problem on my CoCo was the 8vdc as the sound
output on the RCA jack by the composite video out was garbled, everything
else seemed to work OK on the computer, the 5VDC power supply was OK. Try
playing some sound on your CoCo, SOUND 180,10 for example should be clear
and clean, if not then most likely the 8vdc is bad. If sound is OK and the
8vdc supply tests good, check the RF modulator - with the cover off there
is a small solder pad learly labeled +8, 8 volts should be there.

Beyond that you would need a scope to check the cleanliness of the 8v power
supply - the Service Manual states allowed ripple on it. Also a scope would
verify proper video being fed into the RF modulator.


-- RP